In the last years, remarkable research activity has been aimed at developing light emitting devices employed in the optical transmission field within a communication network as light sources.
One type of these devices is the Resonant Cavity Light Emitting Diode (RCLED). U.S. Patent Application Publ. No. 2003/0209714 describes a RCLED including an active layer made up of one or more quantum well (QW) layers containing gallium (Ga), indium (In) and nitrogen (N). The quantum well layer is inserted into a resonant cavity made up of a first mirror of reflecting metallic material which also has the function of electric contact and a second mirror formed by a plurality of pairs of semiconductor layers having different refraction indexes. The materials used for the second mirror are aluminum gallium nitride (AlGaN) and gallium nitride, both with a high N-type doping. This RCLED device is an electric pumping device, and, hence, it provides the application of an electric current passing through the two conductive mirrors causing the optical radiation emission as a consequence of a recombination of charges opposite in sign being formed in the active portion or element.
Vertical Cavity Surface Emitting Laser (VCSEL) devices can be used as an alternative to the RCLED devices. VCSEL devices exploit two conductive mirrors forming a resonant cavity into which an active portion or element (e.g., a laser) that may have a population inversion therein is inserted. Even in this case, the electric pumping is carried out by the electrons passing through the mirrors until they reach the active element. In the case of a VCSEL device, the optical radiation generation is monochromatic and consistent, as a consequence of the optical population inversion in the active element, as it is well-known to those skilled in the art.
A particular VCSEL device is described in the article “Tunable VCSEL”, by C. J. Chang-Hasnain, IEEE J. Select Topics, Quantum Electronics, Vol. 6, No. 6, 2000. With reference to the resonant cavities, both for the RCLED devices and the VCSEL ones, the mirrors are manufactured so as to not have the same reflectivity. Particularly, one of the two mirrors is capable of transmitting a part of the radiation to the outside. For example, a mirror may have an ideal total reflectivity (99-100%) whereas the second one may have a 90% reflectivity, such that the light emission is not symmetric, but, preferably, one-sided.
It shall also be noted that in the prior art resonant cavity devices (RCLED or VCSEL) materials of the III-V groups and the II-VI groups are used for providing the cavities.